1. Field of the Invention
The present invention relates to a single longitudinal mode distributed feedback semiconductor laser and, more particularly, to a distributed feedback semiconductor laser for performing optical feedback by a diffraction grating formed along an optical waveguide.
2. Description of the Related Art
In recent years, a DFB (Distributed Feedback) semiconductor laser has been used as a light source for an optical communication system. This laser includes the diffraction grating along the optical waveguide to realize the single longitudinal operation by the wavelength selectivity. More specifically, the DFB laser using a GaInAsP/InP material is practically used as the light source of long-distance, high-speed optical communication.
In general, a semiconductor laser, e.g., a Fabry-Perot laser, has both facets used as reflection mirrors to provide the optical feedback. In contrast, the DFB laser includes the diffraction grating having the wavelength selectivity to perform the optical feedback. Therefore, the DFB laser may not basically require the optical reflection between the facets. However, in the DFB laser, the single longitudinal mode oscillation depends on the depth and configuration of the diffraction grating, the magnitude of the reflection at the facets, and the phase of the diffraction grating. Therefore, the single longitudinal mode oscillation is not necessarily obtained.
A phase shift DFB laser has been also proposed. The phase shift DFB laser has a structure in which the reflectivity of the cleaved facets is decreased, and the central portion of the cavity has a discontinuous (phase shift) portion (shifted by a phase corresponding to 1/4 the waveguide wavelength .lambda.) of the period of the diffraction grating. Since the laser having the .lambda./4 shift structure has a large oscillation threshold gain difference in the longitudinal mode, it is very advantageous in the single mode operation.
The diffraction grating having the phase shift is usually formed by a two-beam interferometric exposure technique using positive and negative resist materials (e.g., Utaka et al, 29p-ZB-15, Extended Abstract, The 32nd Spring Meeting 1985, The Japan Society of Applied Physics), and the two-beam interferometric exposure technique using a phase mask (e.g., Shirasaki et al, 311 National Conference Record 1984, Semiconductor Devices and Materials, The Institute of Electronics Information and Communication Engineers (IEICE) of Japan) and (e.g., Shirasaki et al, Research Report, OQE85-60, The Institute of Electronics, Information and Communication Engineers (IEICE) of Japan), or the like. In these techniques, it is difficult to optimize reproducibly a kind of resist materials, exposure conditions, design of phase masks, and reduction of unwanted reflection. Therefore, it is difficult to provide the diffraction grating having the phase shift region.
A phase shift DFB laser having a GaInAsP/InP buried heterostructure will be described below with reference to FIGS. 7A and 7B. A first-order diffraction grating 51 is formed on an n-type InP substrate 50, using the two-beam interferometric exposure technique. A .lambda./4 phase shift 52 is then provided in the diffraction grating 51. An n-type GaInAsP waveguide layer 53 (.lambda.=1.3 .mu.m band composition), an undoped GaInAsP active layer 54 (.lambda.=1.55 .mu.m band composition), a p-type InP cladding layer 55 and a p.sup.+ -type GaInAsP ohmic contact layer 56 (.lambda.=1.15 .mu.m band composition) are sequentially grown on the diffraction grating 51, and these layers are mesa-etched in the form of a stripe. Thereafter, a p-type InP buried layer 57, an n-type InP buried layer 58, and an undoped GaInAsP cap layer 59 (.lambda.=1.15 .mu.m band composition) are successively grown on either side of the mesa-stripe to complete the buried structure. A p-type electrode is provided on the top of the structure, and an n-type electrode is formed on the bottom of the structure. At this time, since a current is blocked by a reverse-biased pn junction in the buried regions, the carriers are effectively injected in only the GaInAsP active layer 54. In addition, the reflectivity of the cleaved facets of the structure is decreased by AR (Anti-Reflection) coating 62.
Since the reflection at the facets is greatly lowered in the phase shift DFB laser, the phase of the facets is not almost affected, and single longitudinal mode oscillation is easily obtained. However, the phase shift DFB laser has the following drawbacks.
(1) It is difficult to provide the actual phase shift.
(2) Although the phase shift must be approximately located at the center of the cavity, the position of the phase shift cannot be visually observed after the crystal growth.
In order to eliminate the above drawbacks, there has been proposed a method of forming an equivalent phase shift. According to the method, an equivalent phase shift region having different dimensions such as the width and thickness of the waveguide layer from the peripheral portion is provided instead of forming the actual phase shift in the diffraction grating. Since the equivalent refractive index of the equivalent phase shift region is different from that of the peripheral portion, the phase of the propagating light relative to the grating is changed after passing through the phase shift region. This means that the phase shift is equivalently provided in the diffraction grating. Therefore, the fabrication process requires only the uniformly formed grating without any of actual phase shifts.
A conventional equivalent phase shift DFB laser will be described below with reference to FIG. 8. This laser has an equivalent phase shift region 63 obtained by decreasing the width of the GaInAsP active layer 54 in the central region of the cavity. A stripe pattern mask having a narrow stripe width in the central region of the resonator is formed, and the resultant structure is mesa-etched, thereby easily obtaining the stripe. Particularly such a manufacturing process is not required for a specific process for making the phase shift structure, and the equivalent phase shift structure has characteristics equal to or better than those of a simple phase shift structure (e.g., Matsuyama et al, 30a-SA-17, Extended Abstract, The Spring Meeting 1990, The Japan Society of Applied Physics).
However, in the equivalent phase shift DFB laser, when the thicknesses of the n-type GaInAsP waveguide layer 53 and the undoped GaInAsP active layer 54 vary in every wafers, the variation in the equivalent refractive index may occur. As a result, the length of the equivalent phase shift region 63 for realizing a desired amount of phase shift is changed. Therefore, the amount of the phase shift is varied. In addition, several kinds of masks having different lengths of the phase shift regions are required corresponding to the change in thickness of these layers.
Since a hole burning phenomenon along the cavity direction occurs in the phase shift DFB laser, the single longitudinal mode property is often impaired after the laser oscillation (e.g., Soda et al, Research Report, OQE87-7, The Institute of Electronics, Information and Communication Engineers (IEICE) of Japan). That is, when the value of a normalized coupling coefficient .kappa.L (L: cavity length) is larger than 1.25, guided light is concentrated at the position of the .lambda./4 phase shift 52 (solid line) as shown in FIG. 9A. Such a large deviation in the optical intensity distribution along the cavity direction changes the distribution of the relative carrier density in the GaInAsP active layer 54 (broken line). In addition, the refractive index of the waveguide is changed corresponding to the distribution. Due to the change in the waveguide structure, a large gain difference between longitudinal modes is decreased. That is, the single longitudinal mode property is greatly impaired.
As shown in FIG. 9B, the value of .kappa.L is smaller than 1.25, the relative density of guided light is decreased at the central portion of the cavity (solid line). For this reason, the relative carrier density is increased at the central portion (broken line). Therefore, the refractive index at the central portion is decreased. In this case, since the optimal value (.lambda./4) of the phase shift is canceled, the gain difference is decreased. As a result, the single longitudinal mode property is remarkably impaired.
The effect of the hole burning phenomenon to laser characteristics will be described below. After oscillation is performed, a so-called superlinear curve in which slope efficiency is continuously increased is drawn. Since such a superlinear behavior gives a serious distortion to an output signal, the laser cannot be applied to analog communication systems. A laser having a good linear I-L (current-light output) characteristic is desired. Further, after such a strong nonlinearity, a kink, i.e., a discontinuity in the I-L curve often occurs, resulted in a mode-jump.
Since the equivalent phase shift DFB laser with the narrow width has the reduced optical confinement effect in the equivalent phase shift region to render the relatively small change in the equivalent refractive index, it is not almost affected by hole burning (e.g., Nakano et al, 6p-ZC-7, Extended Abstract, The 49th Autumn Meeting 1988, No. 3, The Japan Society of Applied Physics). However, this effect is not satisfactory.
For compensating the hole burning in the axial direction, a nonuniformed current injection technique has been proposed (e.g., Usami et al, C155, 1988, Autumn National Convention Record, The Institute of Electronics Information and Communication Engineers (IECIE) of Japan). According to the technique, an electrode of the laser is axially divided into a plurality of electrodes which are capable of injecting the current into the active layer separately. The current increasingly injected into the active region having a lower carrier density compensates the nonuniformity to suppress the nonlinearity and the resultant mode jump (caused by the hole burning). However, it is necessary to employ variable resistors corresponding to the number of the divided electrodes. A control system including the adjustment of the variable resistors is complicated, and the cost of the module is increased.
As described above, in the phase shift DFB laser, it is difficult to form the phase shift, and the position of the phase shift cannot be visually observed whether it is located at the cavity center or not. Further, although there is provided the method of forming the equivalent phase shift, it requires several kinds of masks having different phase shift lengths corresponding to change in thickness of the active layer and the waveguide layer. Therefore, such a method takes much time.
In addition, the hole burning phenomenon axially occurs in the phase shift DFB laser, and the single longitudinal mode property is often impaired after oscillation. In the equivalent phase shift DFB laser having the narrow width, the effect of hole burning cannot be sufficiently eliminated. For this reason, the nonuniformed current injection technique is used to compensate the hole burning. However, in order to supply the current to separated electrodes, variable resistors having the same number as that of the divided electrodes are required to complicate the control system.